If you're getting heavily focused, you don't have the GCD to survive 0 healing.

Glyph Inner Fire, and run Inner Fire. You get most of the movement speed from a boot enchant anyways.

Completely wrong.

If you walk out the gates and see any type of double melee cleave you switch to inner fire straight away. Gylph of inner fire is a waste, no respectable priest uses it. Just looking at the ladder the top 5 priests all use Gylph of PoM, and the top priest even uses fear ward over inner fire.

Inner will is used for the insane mana efficiency, when you're casting bubbles / PoMs / renews non-stop it goes a long way and when you are spec'd into it (which most do) it gives you 16% move speed, which is double what the boot enchant does.

I always start the game with Inner Fire until I know what I'm up against, as some enemy rogues will stealth / spring to get an early sap on you. If you're not in Inner Fire you may find things go rapidly downhill if the rogue opens up on you.

I am in Inner Fire 95% of the time in the arena with a few exceptions, the primary one being when the enemy is not attempting to damage me. In that case the extra movement speed is highly useful for making sure I can "pillar hump" to avoid CC effects. The other main exception is when the enemy is moderately damaging me (say, a mage or a warlock) but there's a priest who can mana burn. In that case the movement speed and mana savings trump the damage reduction, as the fight is likely to be won and lost on the mana burn war.